Join the circulatory systems of two mice, one young and one old, a procedure known as heterochronic parabiosis, and the young one suffers some of the impact of aging while the old one loses some of that same impact. Regenerative capacity and stem cell activity are affected, for example. Researchers continue to search for factors in the blood that might explain this, but this work is still in its comparatively early stages. The question of the degree to which the mechanisms involve beneficial factors in young blood or harmful factors in old blood remains to be settled, with a range of interesting evidence on both sides. The "bad old blood" view seems to have the more compelling demonstration so far, I feel. If this is the case, benefits in the old mice are realized because the harmful factors in old blood are diluted by young blood, not because the young blood is providing beneficial signals.
Aging is a gradual biological process characterized by a decrease in cell and organism functions. Gingival wound healing is one of the impaired processes found in old rats. Here, we studied the in vivo wound healing process using a gingival repair rat model and an in vitro model using human gingival fibroblasts for cellular responses associated to wound healing. To do that, we evaluated cell proliferation of both epithelial and connective tissue cells in gingival wounds and found decreased of Ki67 nuclear staining in old rats when compared to their young counterparts.
We next evaluated cellular responses of primary gingival fibroblast obtained from young subjects in the presence of human blood serum of individuals of different ages. Eighteen to sixty five years old masculine donors were classified into 3 groups: "young" from 18 to 22 years old, "middle-aged" from 30 to 48 years old and "aged" over 50 years old. Cell proliferation, measured through immunofluorescence for Ki67 and flow cytometry for DNA content, was decreased when middle-aged and aged serum was added to gingival fibroblasts compared to young serum. Myofibroblastic differentiation, measured through alpha-smooth muscle actin (α-SMA), was stimulated with young but not middle-aged or aged serum both the protein levels and incorporation of α-SMA into actin stress fibers. High levels of PDGF, VEGF, IL-6R were detected in blood serum from young subjects when compared to middle-aged and aged donors. In addition, the pro-inflammatory cytokines MCP-1 and TNF were increased in the serum of aged donors.
In wounds in old rats there is an increased of staining for TNF compared to young rat wounds. Moreover, healthy gingiva (non injury) shows less staining compared to a wound site, suggesting a role in wound healing. Moreover, serum from middle-aged and aged donors was able to stimulate cellular senescence in young cells as determined by the expression of senescence associated beta-galactosidase and histone H2A.X phosphorylated at Ser139. Further, we detected an increased frequency of γ-H2A.X-positive cells in aged rat gingival tissues. The present study suggests that serum factors present in middle-aged and aged individuals may be responsible, at least in part, for the altered responses observed during wound healing in aging.